A number of various types of such trenching machines are known, however, for forming trenches in great depths, only machines which do not have to be hauled from the surface, but have a propulsion system of their own come into consideration. Also, the power supply must take place through electric cables to the trenching machine on the sea bed, where the electric power is transformed to mechanical energy by means of electric motors, the movement of which is transmitted to propelling and trenching means by hydraulic pumps and motors. As a consequence of the electric and hydraulic pumps and motors, the machine will have a considerable weight. Because of the tremendous amount of energy (electrical power) which would otherwise be necessary, units using a mechanical breaking up of the mass to be excavated, must also be employed in trenching machines for use in great depths, whereas the removal of broken masses in admixture with water may take place by suction; such trenching units employing a suction removal of the loosened masses are generally known; see, for example, U.S. Letters Patent to Bates, et al No. 3,717,003. In the patent to Bates, et al, mentioned above, there is disclosed apparatus which is guided along a cable or pipeline, however, the entire weight of the apparatus is imposed on the pipeline or cable as a trench is being formed therealong.
The machine described in the two last-mentioned U.S. patent specifications also have buoyancy tanks reducing the load on the pipeline and maintaining a stable position of the machine relative to a horizontal plane.